1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection circuit on semiconductor chip, and more particular on III/V compound semiconductor chip, wherein a shunt circuit using enhancement-mode HEMT/MESFET technology for protection against ESD is connected between integrated circuit (IC) pad and ground.
2. Description of the Prior Art
An electrostatic discharge (ESD) event is the energy transfer between two bodies at different electrostatic potentials, either through contact or via an ionized ambient discharge (a spark). Therefore, an integrated circuit (IC) connected to external ports is vulnerable to ESD damaging pulses from the operating environment and peripherals. For the fact of ESD event, four models of ESD, Human-Body Model (HBM), Machine Model (MM), Charge-Device Model (CDM), and Field-Induced Model (FIM), are built to present the real damage to IC from ESD event. In order to protect the sensitive IC pad against ESD, an ESD protection circuit has to be connected between main IC pad and its external port. Traditional methods for ESD energy shunting are such as zener diodes, and regular complementary metal oxide semiconductor (CMOS) or bipolar clamp diodes. However, at the much higher data rates of USB 2.0, IEEE 1394, and digital visual interface (DVI) and at radio frequency (RF) operation, the parasitic impedance and capacitance of traditional protection devices can distort and deteriorate signal integrity. The parasitic effect may decrease signal magnitude and magnitude of power amplifier, narrow the applied bandwidth, and decrease the signal to noise ratio. Note that the whole chip space will have a significant increasing, if every port is connected with an ESD protection circuit.
One kind of ESD protection circuit especially for RF component was disclosed by Dr. Sun at U.S. patent application Ser. No. 10/255,787, wherein a transistor for shunting ESD pulse is used. Unlike traditional diode-train-based ESD protection circuit's high turn-ON voltage, it provides various voltage levels for triggering one or more ESD shunt path via one or more small diodes arranged in a series. It rapidly shunts the ESD current via large current carrying transistors in a diode configuration to reduce the impact of the ESD event. Furthermore, it exhibits a reduced parasitic loading effect with the use of small triggering diodes and shunting transistors for the susceptible RF circuit.
In a RF circuit, power amplifier (PA) is the most popular component. The implement of heterojunction bipolar transistors (HBT) for compound semiconductor power amplifier is conventional and also susceptive to parasitic capacitance effect in rf modules. Therefore, Dr. Ma and Dr. Bar disclosed an ESD protection circuit for RF amplifier in InGaP/GaAs HBTs at U.S. patent application Ser. No. 10/501,651, wherein a Darlington pair of HBT instead of the shunting transistor is implemented for shunting large ESD current. For the same epitaxy, the ESD protection circuit with HBT Darlington pair can be integrated with the RF amplifier in InGaP/GaAs HBTs on the same chip. The on-chip ESD protection circuit design can reduce cost, space and power consumption. Furthermore, by arranging more Darlington pairs of HBT in a series, the parasitic effect can be lowered down for not degrading RF and output power performance.
Besides HBT power amplifiers, pseudomorphic high electron mobility transistor (pHEMT) and metal semiconductor FET (MESFET) are also conventional in many implementing RF circuits for their advantages of high signal to noise ratio and low cost. Because their epitaxy growth is totally different with HBT's, the on-chip ESD protection circuit with HBT Darlington pair is not suitable for RF circuit in pHEMTs or MESFETs anymore. Therefore, the shunting transistors in ESD protection circuit has to be replaced by pHEMTs or MESFETs. Note that depletion mode (D-mode) pHEMT/MESFET is a normally on transistor unlike HBTs or bipolar junction transistors (BJTs). In the common operating configuration, event in a negative voltage operation, the D-mode pHEMT/MESFET with its gate coupled to triggering diode string will be conducted and affects the normal of protected circuit. Thus, in the present invention, the enhancement mode (E-mode) pHEMTs or MESFETs are used to replace the shunting transistors in the ESD protection circuit.